This invention relates to a heat displacing type die for extruding a molten resin. Although not limited thereto, the invention is suitable for use as a heat displacing type T die in which a sheet or film of an extruded molten resin is spread transversely to increase its width.
A typical example of the T die is disclosed in U.S. Pat. No. 3,940,221. As shown in FIG. 3, in the T die disclosed therein comprises a lip gap 3 defined between a stationary lip 1 and a flexible or movable lip 2. The lip gap is adjusted by displacing the flexible lip 2 with a bolt 4. The bolt 4 extends through a metal heating block 5 made of steel to reach the flexible lip 2. A cartridge heater 6 is provided on the inner surface of the block 5 for heating the bolt 4 with a cooling passage 7 therebetween. Cooling air is blown into the passage 7 in a direction shown by an arrow. From the inner end of passage the air flows transversly. As a consequence, when the bolt 4 is heated by the cartridge heater 6, the bolt 4 elongates to narrow the lip gap 3 by pushing down the flexible lip 2. Conversely, as the bolt 4 cools, the lip gap 3 is widened. Where the width of the extruded resin sheet is large, a plurality of T dies are juxtaposed side by side.
The lip gap adjustment is possible even with the prior art heat displacing type die. However, since this prior art device is constructed to vary the lip gap by utilizing the thermal expansion and contraction of the bolt 4 so as to adjust the flow quantity of a molten resin, a substantially large temperature difference, 100.degree. C. or more, would be created among a plurality of juxtaposed bolts. Such temperature difference has a more or less effect upon parts other than the bolts. Particularly, the temperature difference has an influence upon the viscosity of a molten resin extruded through the lip gap. For this reason, a heat insulating layer 8 is provided between the heating block 5 and the upper die block 2A formed with the flexible lip 2 for preventing heat transmission from the bolt 4 to the molten resin. However, perfect interruption of such heat transmission is actually impossible. Moreover there are other heat transmission paths, for example from the bolt 4 to the flexible lip 2. Thus it is impossible to prevent variations in the temperature and viscosity of the molten resin.
When the bolt is caused to expand by heat, although the lip gap is narrowed to decrease the flow quantity of the molten resin, the heat is transmitted through various heat transmitting routs to decrease the viscosity of the molten resin, thus increasing its flow quantity. Conversely when the bolt shrinks, the lip gap is increased to increase the flow quantity. On the other hand, the viscosity of the molten resin increases thus causing a decrease in the flow quantity. In this manner, the variation in the lip gap and the variation in the flow quantity caused by the viscosity variation contradict with each other. For example, even when the lip gap is decreased for the purpose of decreasing the flow quantity of the molten resin the resin viscosity tends to decrease thereby increasing the flow quantity thus failing to adjust the lip gap as desired. The same problem occurs when the lip gap is increased.